The Natural Carbon Cycle

The concentration of CO2 is controlled by a variety of processes that add and subtract CO2 to and from the atmosphere. Nearly all of these processes are cyclic - for example, the removal of CO2 by plant photosynthesis,

is balanced by the return of CO2 and the consumption of oxygen (O2) when plant tissues burn or decompose:

The global carbon cycle consists of a variety of such balanced processes operating at different rates and different timescales. The cycles are overlaid on one another, each contributing to the overall, global biogeochemical cycle of carbon.

The most basic cycle, often called the carbonate-silicate subcycle, is driven by the reaction of atmospheric CO2 with the Earth's crust, causing the chemical breakdown of rocks, known as rock weathering. Since this reaction would occur even on a lifeless Earth, it is a component of the abiotic carbon cycle on Earth (Fig. 1). Rock weathering transfers CO2 to the world's oceans, via rivers, in the form of bicarbonate (HCOi"). Bicarbonate is eventually removed from seawater by the deposition of calcium carbonate (limestone, or CaCO3), which is added to Earth's oceanic crust. When the oceanic crust undergoes subduction and heating under great pressure (i.e., metamorphism), CO2 is returned to the atmosphere in volcanic emanations. The presence of life on Earth has increased the rate of some of these processes (e.g., witness the deposition of marine carbonate by oysters), but the carbonate-silicate cycle appears to have turned slowly for nearly all of geologic time. Very few marine sediments are more than 150,000,000 years old (Smith & Sandwell, 1997). Presumably, the carbon content of older sediments has been returned to the atmosphere.

Each year, the amount of carbon moving in the carbonate-silicate cycle is relatively small: volcanic emissions are currently estimated between 0.02 and 0.05 Pg C/year (Bickle, 1994; Williams, Schaefer, Calvache, & Lopez, 1992), annual river flow of HCO^ is 0.40 Pg C/year (Suchet & Probst, 1995), and the formation of CaCO3 carries about 0.38 Pg C/year to ocean sediments

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